1. Field of the Invention
Eyeglass lenses are initially manufactured as circular, elliptical or other shape blanks which are ground to size to fit the rings or surrounds of the eyeglass frames to which they are to be fitted.
2. Description of the Prior Art
They are usually trimmed to size on a grinding machine.
At present the grinding machine usually includes at least one grinding wheel rotatably mounted on a frame, a carriage pivoted to the frame to pivot about an axis parallel to the rotation axis of the grinding wheel, a support spindle rotatably mounted on the carriage to rotate about an axis also parallel to the rotation axis of the grinding wheel and which is adapted to receive axially the lens to be ground and a template, and a feeler in vertical alignment with the template and in practise mounted on the frame so that it is movable transversely relative to the pivot axis of the carriage.
The feeler is part of what is usually called a vernier head, forming the part of the latter with which the template cooperates, and it is the feeler which controls the grinding operation.
When the template is circular and is held in contact with the feeler by the weight of the carriage alone or by the combined effect of this weight and ancillary means such as springs or counterweights so that the bearing force can be adjusted as necessary, the feeler is under the control of control means which in conjunction with data on the contour of the rings or surrounds of the eyeglass frames to be equipped with the lens command a displacement causing the carriage to pivot in one direction or the other to vary the trimming accordingly at the required locations.
A proposal has been made to implement the feeler by means of strain gauges, as for example in French patent application No 2 652 775 (filed under application No 89 13031 on 5 Oct. 1989).
In practise this presupposes the intervention of an elastically deformable member and taken overall the resulting component is therefore costly, fragile and difficult to use.
Another proposal is to implement the feeler as a displacement sensor, for example an opto-electronic cell, operative between a pad on which the edge of the template rests and a plate on which the pad is mounted to pivot about an axis parallel to the pivot axis of the carriage.
The tripping threshold of the opto-electronic cell is very precise, however, and therefore difficult to adjust.
Also, the inevitable assembly clearance at the articulation between the pad and the plate is likely to introduce some degree of inaccuracy into the reference point that the combination is supposed to provide, resulting in distortion in the shape of the eyeglass lens produced.
A general object of the present invention is a grinding machine which does not have these drawbacks.